1. Field of the Invention
This invention relates to an automatic-transmission control system.
2. Description of the Prior Art
There have been known various automatic-transmission control systems which electronically control the gear-shifting operation of an automatic transmission by the use of a microcomputer. For example, the automatic-transmission control system disclosed in Japanese Unexamined Patent Publication No. 62-93552 causes the automatic transmission, when a jump shifting from a certain gear speed to a gear speed which is higher or lower than the gear speed by two or more stages is required, to shift once to an intermediate gear speed and then shift to the final gear speed in order to suppress shift shock.
Generally a jump shifting is required when the throttle opening is abruptly reduced in a state in which the engine load has been heavy and the throttle has been wide opened, and usually the jump shifting is 1-3 upshift or 2-4 upshift. If the automatic transmission directly shifts to the highest gear speed, the gear ratio changes largely and the engine speed abruptly lowers, which gives rise to a shift shock. In order to avoid such a shift shock, the aforesaid automatic-transmission control system causes the automatic transmission to shift once to an intermediate gear speed and then shift to the highest gear speed when a jump shifting is required. When the engine load is heavier than a predetermined value, the automatic transmission control system allows the automatic transmission to directly shift to the highest gear speed.
However the automatic-transmission control system, which controls the automatic transmission in the manner described above when the jump shifting is required, is disadvantageous in that, since the automatic transmission shifts directly to the highest gear speed when the engine load is very heavy, friction coupling elements in the automatic transmission are subjected to heavy load and the durability of the automatic transmission lowers, and in that, since the automatic transmission once shifts to an intermediate gear speed when the engine load is relatively light, driver's power requirement cannot be satisfied.
For example, when the accelerator pedal is released and the gear speed determined on the basis of the shift pattern becomes third or fourth in a state where the engine load has been very heavy and the engine output power has been maximized with the automatic transmission in first or second, heavy load transitionally acts on the friction coupling elements to produce wear and heat in the case the engine load at that time is still heavy, thereby promoting deterioration of the durability. On the other hand, when the automatic transmission is caused to shift to an intermediate gear speed before shifting to the final gear speed in order to reduce the load which acts on the friction coupling elements, driver's power requirement cannot be well satisfied since the gear speed which conforms to the requirement is the final gear speed.
Further, when the automatic transmission shifts to the intermediate gear speed with the vehicle speed higher than a predetermined value, the engine speed will become excessively high, which adversely affects the durability of the engine.